The invention relates to trimming an integrated circuit, such as a silicon temperature sensor.
The electrical characteristics (e.g., bias voltages and currents) of an integrated circuit typically are functions of parameters of a process that is used to fabricate the circuit. The parameters might include resistivities and threshold voltages, for example, and the process might be, for example, a CMOS process. Engineers typically take into account ideal parameters of the process when designing and laying out the integrated circuit.
The actual electrical characteristics of the circuit should match the designed characteristics. However, quite often, process fabrication variations cause the actual parameters of the process to vary. As a result, the actual electrical characteristics of the integrated circuit are different from the designed characteristics. Although some circuits are designed to have a minimum sensitivity to process variations, other circuits are by their very nature quite sensitive to process variations.
After fabrication, a technique called trimming is typically used to compensate for process variations. For example, the resistance of an integrated resistor (e.g., an n-well resistor) might vary up to fifty percent from the designed value. Referring to FIG. 1, to adjust, or trim, the resistance of a resistor 2 after fabrication, quite often a nonlinear trimming technique is used to adjust the effective resistance of the resistor by selectively coupling trimming resistors 3 (via transistors 4) in parallel with the resistor 2. However, a difficulty with this technique is that the trimming is nonlinear. Therefore, to properly trim the resistor 2, a considerable number of resistors 3 might be needed, and thus, a large amount of die space may be consumed for trimming purposes.
Because each resistor 3 generally consumes a considerable amount of die area (versus a transistor, for example), the above-described trimming technique is quite often limited by the available area in the die. Furthermore, it is quite often difficult to trim the resistance near the desired valve due to the large tolerances of the resistors 3 and the resistance introduced by the channel resistance of the transistor 4. The channel resistance of the transistor 4 is a function of a threshold voltage (typically referred to as V.sub.T) which is a process parameter that is also subject to variation.
Referring to FIG. 2, an example of a circuit that is typically sensitive to process variations is a thermal sensor 8. The sensor 8 might, for example, monitor a substrate temperature of a microprocessor. When the temperature exceeds a predetermined threshold temperature (e.g., 100.degree. C.), the sensor 8 alerts other circuitry of the microprocessor so that corrective action (e.g., throttling back or shutting down of the microprocessor) may be taken to reduce the temperature. Without the corrective action, the substrate may overheat, and catastrophic failure of the microprocessor may occur.
The sensor 8 typically must precisely sense the temperature to avoid generating false alarms when the temperature is below the predetermined threshold temperature. These false alarms might slow down or shut down a microprocessor that is otherwise behaving normally.
For purposes of determining when the threshold has been exceeded, the thermal sensor 8 typically has a comparator 10 that electrically compares the temperature of the substrate to the threshold. To accomplish this, the comparator 10 receives a thermal trip point signal (called V.sub.REF) that electrically represents the predetermined temperature threshold. The comparator 10 also receives a signal (called V.sub.SENSE) that represents a measured temperature of the substrate. The comparator 10 compares the V.sub.REF signal which typically has a positive temperature coefficient with the V.sub.SENSE signal which typically has a negative coefficient. Typically, the V.sub.SENSE signal is furnished by a diode.
The DC voltage level of the V.sub.REF signal typically represents the predetermined temperature threshold. However, this voltage level is sensitive to process variations. As a result, the V.sub.REF signal may not accurately represent the threshold, and thus, the overall accuracy of the sensor 8 may be limited by the process variations.
Thus, there is a continuing need for an integrated circuit that allows accurate, post fabrication trimming of the circuit.